FR2725791A1 - METHOD AND APPARATUS FOR DETERMINING NOISE FACTOR OF OPTICAL AMPLIFIER - Google Patents

Abstract

An optical amplifier system includes an optical fiber doped with a rare earth and a continuous laser source (1) (A) to excite atoms in the fiber to a higher energy level. According to the invention, the source (1) is modulated using a first pulse cycle markedly shorter than the lifetime of the atoms in order to generate a pulse signal (C) to the amplifier, a output signal (D) of said amplifier using the pulse and generating a second pulse having an increased phase differential with respect to the first pulse, maximum and minimum output electric powers are obtained from a series of amplifier output signals and the noise figure is determined on the basis of these powers.

Description

NOISE FACTOR DETERMINATION METHOD AND APPARATUS

OPTICAL AMPLIFIER

  The present invention relates generally to the measurement of a noise factor and relates in particular to a method and an apparatus for measuring noise factor which allows simple, rapid and

precise phase differential.

  Currently, two types of optical amplifiers are known: those based on optical fibers doped with an element chosen from rare earths such as erbium (Er), and those based on a semi-optical amplifier. driver. A parameter for characterizing the performance of these optical amplifiers is the noise factor, and a method and apparatus for determining the noise factor (NF) have been developed and described in the Japanese patent application.

bearing the number H6-17 593.

  In the case of the optical amplifier based on optical fibers doped with erbium (hereinafter abbreviated as optical fibers), when an excitation laser light is injected inside the optical fiber, certain atoms located in the optical fiber are excited from a basic energy level to a higher energy level. When the atoms return from the higher energy level to the basic level, light is emitted in the process. There are two modes of light emission: a first is an induced emission process which occurs due to the atomic transition action induced by the excitation laser light, and another is a process

  spontaneous emission (hereinafter called SE process).

  Optical amplification is obtained through the induced emission process. The transition stage (from a high energy level to a low energy level) in the induced emission process occurs quickly, however, the transition stage from spontaneous emission occurs slowly with respect to speed of the induced emission process. In optical fibers doped with erbium, the lifetime of the spontaneous emission (that is to say the lifetime of the excited atoms) lies in a range from several milliseconds to several tens of milliseconds. The spontaneous light (hereinafter called SE light) generated in the spontaneous emission process is amplified inside the optical fiber doped with erbium and is emitted in the form of amplified spontaneous light (called

below ASE light).

  The noise factor (NF) of an optical amplifier is determined as follows. An object optical amplifier whose NF must be determined is irradiated using a pulsed laser light having a pulse cycle significantly shorter than the lifetime of the spontaneous emission, and the resulting values of the energy. outgoing electric, with and without laser excitation, are measured. That is to say that on the one hand the PASE transmitted power generated during the time interval of a spontaneous emission is measured (without laser excitation), and on the other hand the sum of the output power is measured PAMP laser signal and the PASE power output (i.e. PAMP. + PASE) during the laser excitation interval. Then, the noise factor is given by the following equation: NF = (PASE / hxux A x BO) + 1 / A ... (1) in which h is the Planck constant, u is the optical frequency of the laser light entering the object optical amplifier, A is the gain of the object optical amplifier, and B0 is the width of the transmission band of the device for measuring the

  PASE emitted power of ASE light.

  The gain A of the optical amplifier can be approached by the following equation: A t (PAMP - PASE) / PIN ... (2) in which PIN is the value of the input power of the laser light in the object optical amplifier. A first problem encountered in the method for determining the NF presented above is due to the fact that the pulsed laser light introduced into the object optical amplifier must first propagate through the optical amplifier before being extracted from the optical fiber, and therefore the laser output is delayed, the delay time being a function of the length of the optical fiber. It follows that object optical amplifiers having different lengths have different delay times. An additional delay is caused by the optical fibers connecting the object optical amplifier and an NF determination system. Therefore, in this approach for determining the NF of an optical amplifier, it is required that the various delay times are predetermined and that the phase relationship of the switched laser pulses is determined between the phases of pulses measured both during time intervals with

and without pulsed laser excitation.

  It is an object of the present invention to provide a rapid and precise method for adapting the delays of the output pulses of an optical amplifier system and its associated optical fibers, and to present an apparatus for determining the noise factor. an optical amplifier system based on optical fibers doped with a rare earth

  to enter / exit optical signals.

  The object is achieved in a method for determining the noise factor of an optical amplifier system comprising an optical fiber doped using an element chosen from rare earths and a continuous laser source (cw) intended for providing an atomic excitation causing the atoms located in said doped optical fiber using an element chosen from rare earths to be excited at a higher energy level, comprising the steps consisting in: modulating said continuous laser source using a first pulse having a cycle significantly shorter than the lifetime of said atoms to generate an optical pulse signal intended to be sent to an optical amplifier whose noise factor is to be determined, synchronize a signal of optical output from said optical amplifier using said pulse and generating a second pulse having an increased phase differential with respect to said pr first pulse, obtain a maximum electrical output power and a minimum electrical output power from a series of optical output signals emitted by said optical amplifier by successively modulating said optical output signals using a second successive pulse, and determining said noise factor based on said maximum electrical output power and said minimum electrical output power. It is also an object of the present invention to provide an apparatus, suitable for the above method, comprising: pulse generator means for generating a first pulse having a cycle significantly shorter than the lifetime of said atoms and a second pulse having an increased phase differential with respect to said first pulse, optical pulse generating means for generating an optical pulse signal continuously by modulating said continuous laser source using said first pulse and sending said optical pulse signal to an optical amplifier, modulation means for modulating an optical output signal from said optical amplifier, using said second pulse, electrical power measurement means intended to measure the value an electrical output power produced by said modulation means, and control means for storing the successively measured values of said electrical output power, controlling the output of said second pulse generated by said optical pulse generating means and determining said noise factor on the basis of the maximum value of said electrical output power and the value

  minimum of said electrical output power.

  In accordance with the method and the apparatus presented above, the pulse generator device is arranged to produce first pulses whose cycle is significantly shorter than the lifetime of the excited atoms generated in the doped fiber. The pulse generator device also produces second pulses whose phase differentials increase successively up to a given phase differential which is clearly below

  of the lifetime of the excited atoms.

  The optical pulse generator device is arranged to produce optical pulses by modulating the continuous laser using the first pulses, and applies the optical pulses to

  the NF noise factor determination device.

  The modulation device is arranged to modulate the optical output signals from the device using the second pulses to produce other differentials in accordance with the requirements of the device system and its devices

auxiliaries.

  The electrical power measurement device is arranged to measure the electrical power of the signal emitted by the modulation device, and sends the

  results measured in the control device.

  The control device is arranged to store the successive values of the measured electrical power and to control the output values of the second pulses generated by the optical pulse generator device as well as to calculate the noise factor of the device using the maximum and minimum values of the

electrical power measured.

  The object is also achieved in a variant of the method comprising the steps consisting in: generating an optical pulse signal by modulating said continuous laser source using a first pulse having a cycle markedly shorter than the lifetime of said atoms to generate an optical pulse signal intended to penetrate an optical amplifier whose noise factor is to be determined, synchronize an optical output signal from said optical amplifier using said first pulse to generate a second pulse having an increased phase differential with respect to said first pulse, obtaining a maximum electrical output power and a minimum electrical output power from a series of said optical output signals emitted by said optical amplifier by successively modulating said optical signals output using a second successive pulse, choose from second pulses a third pulse giving a small duty cycle to said maximum electrical output power to define a first electrical output power, choosing from the second pulses a fourth pulse giving a small duty cycle to said minimum electrical power output to define a second electrical output power, and determining said noise factor based on said first electrical output power and said second electrical output power. This basic method and apparatus allow a precise and rapid determination of the noise factor allowing the delay effects caused by the internal and external doped optical fibers associated with

  the noise determination device.

  The object is achieved in a variant of the device comprising: pulse generator means for generating a first pulse having a cycle significantly shorter than the lifetime of said atoms and a second pulse having an increased phase differential compared to said first pulse, optical pulse generating means for continuously generating an optical pulse signal by modulating said continuous laser source using said first pulse and sending said optical pulse signal to an optical amplifier, modulation means for modulating an optical output signal from said optical amplifier using said second pulse or using a pulse to produce a smaller duty cycle relative to said second pulse, measuring means of electrical power for measuring the value of the electrical output power produced by said modulating means ion, and control means for successively storing the measured values of said electrical output power, choosing a third pulse from the second pulses giving a small cyclic ratio to said maximum electrical output power to define a first electrical power of output, select a fourth pulse from the second pulses giving a small duty cycle to said minimum electrical output power to define a second electrical output power, and determine said noise factor based on said first electrical output power and said second electrical output power. According to the variant of the method and apparatus presented above, the pulse generator device is arranged to produce first pulses whose cycle is significantly shorter than the lifetime of the excited atoms generated in the doped fiber. The pulse generator device also produces second pulses whose phase differentials increase successively up to a given differential which is clearly below the lifetime of the excited atoms, or pulses whose cyclic ratio is smaller compared to on the second pulse. Therefore, there is no distortion problem caused by the modulation of the light

  laser using these input pulses.

  The optical pulse generator device is arranged to produce optical pulses by modulating the continuous laser using the first pulses, and applies the optical pulses to

the NF determination apparatus.

  The modulation device is arranged to modulate the optical output signals from the apparatus using the second pulses or pulses whose cyclic ratio is smaller compared to the second pulses to produce optical signals having differentials of phase corresponding to the requirements of the device and its

auxiliary devices.

  The electric power measurement device is arranged to measure the electric power of the signal emitted by the modulation device and sends the

  results measured in the control device.

  The control device is arranged to memorize the successive values of the electrical power measured, to choose from the second pulses a third pulse whose cyclic ratio is small compared to a maximum electrical power measured and to send the pulse to the device pulse generator, for choosing from the second pulses a fourth pulse whose cyclic ratio is small compared to a minimum measured electrical power and for sending the pulse to the pulse generator device, and for calculating the noise factor of the device using a first electrical power produced by the third pulse acting on the modulation device and a second electrical power produced by the fourth pulse acting on the modulation device. This approach allows a more precise determination of the noise factor, allowing the effects of

measurement errors.

BRIEF DESCRIPTION OF THE DRAWINGS

  Figure 1 is a block diagram of an apparatus according to the present invention for determining a

  optical amplifier noise factor.

  Figure 2 is a first example of the synchronization of pulse waveforms in various

sections of the device.

  Figure 3 is a second example of the synchronization of pulse waveforms in various

sections of the device.

  FIG. 4 is a graph showing the relationship between the electrical power emitted and the successive delay times in the

  determination of NF of the present invention.

  FIG. 5 is an example of the effect of the change in duty cycle of the pulse G on the synchronization of the pulses in various sections of the apparatus. There will be described, with reference to FIGS. 1 to 5, a preferred embodiment of the present invention of a method for determining NF and of a

  device intended to be used therewith.

  Figure 1 is a block diagram intended to represent the main components of the NF determination apparatus, and Figures 2 and 3 represent various pulse waveforms in the respective sections of the apparatus. Referring to Figure 1, laser light A from a continuously working laser source 1 (cw) having a band of 1.55 µm is introduced into an optical switch 2. The optical switch 2 converts the laser light A as an optical pulse signal C having a cycle of 1 us and a duty cycle of 50%, in accordance with a pulse B supplied by a section 3 of the control device, and sends the optical signal C into an object optical amplifier. The optical amplifier object 4 comprises an optical fiber doped with Er at an excitation level of 1.48 in and amplifies the optical signal C and emits an optical signal D. As shown in the waveform (i) on the FIG. 2, the cycle of the pulse B is chosen to be 1 ps, clearly shorter than the lifetime of spontaneous emission of the optical fiber doped with Er (from several ms to several tens of ms). As can be seen in the waveform (ii) of FIG. 2, the optical pulse signal C has a time interval Tl during which the laser excitation is supplied and a l time interval T2 during which no laser excitation is not provided. As shown in the waveform (iii) in FIG. 2, the optical signal D is emitted with a time delay Ta which is a function of the length of the optical fiber doped with Er and other factors causing a delay in the optical amplifier object 4. During the time interval T1, the optical signal D consists of amplified laser light and of the light ASE while during the time interval T2, the optical signal D consists only ASE light. In other words, the electrical output power during the time interval T1 consists of a sum (PAMP + PASE) consisting of the electrical output power PAMP generated by the amplified laser light and the electrical output power PASE from the ASE light while the electrical power during the time interval T2 consists only of the PASE power generated by the ASE light. An optical switch 5 modulates the optical signal D in accordance with a pulse G emitted by the control section 3 and sends an optical signal H. A section 6 of optical / electrical conversion (O / E) chooses an electrical output power PO coming from the optical signal

  input H and sends PO in the control section 3.

  We will now describe the operation of

  the noise factor determination apparatus.

  When the measurement start switch is pressed, the control section activates the continuous laser source 1, and via a pulse generator device and an optical pulse generator device, generates a pulse optical B shown in FIG. 2, towards the optical switch 2. From this moment, the pulse B is transmitted continuously to the optical switch 2 which continuously transmits an optical pulse signal C (see FIG. 2 ) up to the interior of the object optical amplifier 4. The result is that the optical signal D is emitted continuously by the object optical amplifier 4. The control section 3 then emits a pulse signal G1 which is generated by delaying the pulse B by 15.625 ns, towards the optical switch 5. This effect is represented in FIG. 3. At this moment, the control section 3 sends instructions to its internal memory to memorize the signal output from section 6 of optical / electrical conversion O / E. The control section 3 then emits a pulse signal G2 (see FIG. 3) which is generated by delaying the pulse signal G1 by 15.625 ns, to the optical switch 5, and again sends instructions to its internal memory for memorize the output signal from the optical / electrical O / E conversion section 6. This sequence of events (consisting in delaying all successive signals by 15.625 ns and in sending the delayed signal to the optical switch 5) continues until a pulse signal G64 is generated. Similarly, the signals transmitted by the O / E conversion section 6 are successively stored in the internal memory.

of the control section 3.

  The cumulative delay time over the 64 delay events is 1 ps (15.625 ns x 64), in other words, the optical switch 5 successively produces a series of 64 pulse signals G each having a time of minimum delay consisting of 1/64 of a cycle to achieve a maximum cycle delay time of 1 ps. Each of the pulse signals G input into the optical switch 5 produces a corresponding electrical output power. The values of the output power PO from the optical switch 5 are measured for each delay time, producing

  the 64 successive measurements shown in Figure 4.

  By designating by Tmax the delay time (measured with reference to pulse B) of a pulse signal G to produce the maximum electrical output power and by Tmin the delay time of a pulse signal G for produce the minimum output electrical power, we can see by comparing the waveforms (iv) and (v) in Figure 2, that: Tmax - Ta, and that

Tmin t Ta + T1.

  The process outlined above makes it possible to provide the delay time Ta for the optical signal D in

as long as Tmax.

  Then, the control section 3 generates a pulse signal Si, represented by a waveform (iii) in FIG. 5, having a cycle time of 1 ps, a pulse width of (0.5 - 2 Td) ps and whose rise is delayed compared to that of the pulse B of (Tmax + Td) us, where Td represents a rise delay time. The pulse signal Si is then sent to the optical switch 5 and the value of the electrical output power (indicated by Pi) coming from the O / E conversion section 6 is stored in the internal memory. The amplitude of the rise delay time Td is much shorter than the cycle time of 1 pS of the pulse signal Si, and is chosen according to the measurement error. The control section 3 then emits a pulse signal S2 (called waveform (iv) in FIG. 5) which is the pulse signal Si delayed by 0.5 ls and sends the pulse signal S2 to the optical switch 5. The value of the electrical output power (indicated by P2) coming from the O / E conversion section 6 is stored in the internal memory. The output values P1 and P2 replace PAMP and PASE, respectively in equation (2), and the noise factor (NF) is

  calculated from equations (1) and (2).

  It should be noted that although it is possible to calculate NF using the maximum and minimum values shown in Figure 5, such a method offers the possibility that the calculation results can

contain measurement errors.

  Although the method and the apparatus for determining NF have been explained with reference to the specific examples of embodiment above, it is clear that other modifications of the apparatus are possible, than other circuit configurations. can be carried out while remaining in the principles of phase adjustments of the input and output pulses using a synchronized pulse and a compensating return pulse from a control section to an optical amplifier so that the determination of NF can be implemented without having the phase delay problems caused by a delay

  propagation in optical fibers.

Claims (4)

  1. Method for determining noise factor in an optical amplifier system comprising an optical fiber doped using an element chosen from rare earths and a source (1) of continuous laser (A) intended to provide a atomic excitation causing the atoms located in said optical fiber doped using an element chosen from rare earths to be excited at a higher energy level, characterized in that it comprises the steps consisting in: modulating said continuous laser source using a first pulse having a cycle significantly shorter than the lifetime of said atoms to generate an optical pulse signal (C) intended to be sent into said optical amplifier (4), the noise factor is to be determined, synchronize an optical output signal (D) from said optical amplifier using said first pulse and generate a second pulse having a phase differential ac raw with respect to said first pulse, obtaining a maximum electrical output power and a minimum electrical output power from a series of optical output signals emitted by said optical amplifier by successively modulating said optical output signals at the using a second successive pulse, and determining said noise factor on the basis of said maximum electrical output power and said power minimum output electric.   2. Method for determining the noise factor in an optical amplifier system comprising an optical fiber doped using an element chosen from rare earths and a source (1) of continuous laser (A) intended to provide a atomic excitation causing the atoms located in said optical fiber doped using an element chosen from rare earths to be excited at a higher energy level, characterized in that it comprises the steps consisting in: modulating said continuous laser source using a first pulse having a cycle significantly shorter than the lifetime of said atoms to generate an optical pulse signal (C) intended to be sent to an optical amplifier (4) whose noise factor to be determined, synchronize an optical output signal (D) from said optical amplifier using said first pulse and generate a second pulse having an increased phase differential with respect to said first pulse, obtaining a maximum electrical output power and a minimum electrical output power from a series of said optical output signals emitted by said optical amplifier by successively modulating said optical output signals at the using a second successive pulse, choose from the second pulses a third pulse giving a small duty cycle to said maximum electrical output power to define a first electrical output power, choose from the second pulses a fourth pulse giving a small duty cycle to said minimum electrical output power to define a second electrical output power and determine said noise factor based on said first electrical output power and said second electrical output power. 3. Apparatus for determining a noise factor for an optical amplifier system comprising an optical fiber doped using an element chosen from rare earths and a source (1) of continuous laser light to generate an atomic excitation entraining the atoms located in the optical fiber doped with an element chosen from the rare earths to be excited at a higher energy level, characterized in that it comprises: means (3) pulse generators for generating a first pulse having a cycle significantly shorter than the lifetime of said atoms and a second pulse having an increased phase differential with respect to said first pulse, optical pulse generating means for generating an optical pulse signal so continuously by modulating said source (1) of continuous laser using said first pulse and sending said optical pulse signal to an amplifier o ptic (4), modulation means for modulating an optical output signal from said optical amplifier using said second pulse, electrical power measurement means for measuring the value of an electrical output power produced by said modulation means, and control means (3) for memorizing the successively measured values of said electrical output power, controlling the output of said second pulse generated by said optical pulse generator means and determining said noise factor on the base of the maximum value of said electrical output power and the value   minimum of said electrical output power.   4. Apparatus for determining a noise factor for an optical amplifier system comprising an optical fiber doped using an element chosen from rare earths and a source of continuous laser light to provide atomic excitation driving the atoms located in the optical fiber doped using an element chosen from the rare earths to be excited at a higher energy level, characterized in that it comprises: pulse generator means for generating a first pulse having a cycle significantly shorter than the lifetime of said atoms and generating a second pulse having an increased phase differential with respect to said first pulse, optical pulse generating means for generating an optical pulse signal, continuously, by modulating said continuous laser source using said first pulse and sending said optical pulse signal to an opt amplifier a modulation means for modulating an optical output signal from said optical amplifier, using said second pulse or a pulse to produce a smaller cycle ratio with respect to said second pulse, measuring means of electrical power intended for measuring the value of an electrical output power produced by said modulation means, and control means for storing the successively measured values of said electrical output power, choosing a third pulse from second pulses giving a small cycle ratio to said maximum electrical output power to define a first electrical output power, choose a fourth pulse from second pulses giving a small cycle ratio to said minimum electrical output power to define a second electrical power output and determine said fuzz factor it based on said first electrical output power and said second electrical output power.